1. Field of Invention
This invention relates to a novel and improved method of making an anode rod assembly for the cathodic protection of metal surfaces, such as the internal ferrous metal wall surfaces of water heaters.
Metal surfaces, particularly those of ferrous material, when exposed to air and water, undergo oxidation or corrosion. Ferrous metal surfaces, such as the internal surfaces of water heater tanks, have been coated with other metals, such as zinc (galvanized), to protect the surfaces against corrosion and thereby prolonging the life of the tank. Glass lining has been used for this purpose, and, at present, almost all water heater tanks are glass lined. However, even the glass lining per se is not sufficient to afford a satisfactory long life to water heater tanks. No lining is perfect, and any defect or imperfection is a site for corrosion. Consequently, manufacturers rely upon anodes of a galvanically active metal, i.e., sacrificial anodes, to afford additional corrosion protection to the tank.
The conventional method of making an anode rod of a galvanically active metal, e.g., magnesium, aluminum, zinc and their respective alloys, is to extrude or cast the metal around a core wire or rod of another metal, generally a ferrous metal. The anode rod is disposed in a tank and fastened by a suitable means to the tank to provide a mechanical and electrical contact between the anode and the tank. In the generally used technique, the anode rod is threaded at one extremity and fitted into an internally threaded cap member, which is generally a ferrous material; however, other metals, such as brass, could be used. The cap member, in turn, is threaded by means of external threads into a suitable threaded coupling or spud located in a wall of the tank.
2. Description of Prior Art
It has been standard prior art practice to position anode rod assemblies in tanks, such as water heaters, usually by means of a mated threaded joint between an anode rod cap member and a suitable threaded member integral with or fixedly attached to a tank wall. Examples of these are shown in U.S. Pat. Nos. 3,558,463; 2,568,594; 3,037,920; 3,542,663 and 2,459,123. These references further show typical means for joining the anode rod to the cap member, e.g., by a threaded joint, by adhesively bonding, by a crimping operation, or by riveting or welding.
Additionally, there are references in the general metal-working field which show the fastening of metal elements wherein one member is in the shape of a rod by means of crimping, bulging or deformation. Exemplary of these are U.S. Pat. Nos. 1,776,615; 2,434,080; 2,896,981; 3,209,437 and 3,638,505. These references, although useful as general background information in metal working or forming, neither disclose nor suggest the novel method of manufacturing the sacrificial anode assembly of the instant invention whereby there is produced a joint between an anode rod and a cap member which not only has superior mechanical and electrical characteristics but also exhibits superior corrosion resistance in the zone of the joint, a property which ensures a long effective life to a sacrificial anode rod assembly when exposed to a corrosive environment, such as that experienced in a water heater tank.
In the joining of anode rods to cap members as disclosed in the prior art, there have been certain deficiencies which have detracted from their rendering a long service life. For example, in the methods of the prior art, machining and/or threading of an anode rod extremity is done and this removes a significant amount of anode rod metal.
Further, this removal of the anode metal is at a position on the anode rod where, upon installation in a water heater, the anode rod would be subjected to excessive attack. Adhesive joining of the anode rod to the cap member has deficiencies as a joining method because if the adhesive fails, the connection between the anode rod and the cap member fails. Riveting and welding of anode rods to cap members, which are of dissimilar metals, such as is shown in U.S. Pat. No. 2,459,123, have inherent drawbacks in obtaining sound mechanical and electrical joints that will withstand the corrosive attack to which they will be exposed.